The present invention relates to a new and improved circuit for transferring measured values from an incremental position measuring system, the analogue output signals of which are digitized and transferred via a counter to a computer which transmits them to an evaluation device.
With a prior art circuit of this general type, as disclosed in U.S. Pat. No. 4,229,646, granted Oct. 21, 1980 the position measuring system is formed by a scale and a scanning head, and by an electronic up-down or forward-backward counter. This position measuring system serves for measuring or positioning the relative location or orientation of two objects which can be, for instance, the carriage and the bed, respectively, of a processing or measuring machine. The analogue output signals delivered by the scanning head are first converted into square-wave pulse-shaped digital signals and then transferred to the counter. The counter is connected to a digital computer which compares the counted values with values received from a reference or set point storage and transmits the difference to a display or indicator device. This display or indicator device is read by an operator who either checks by means of the display whether or not the relevant machine has reached the desired position, or else the operator manually intervenes in the positioning operation upon reading the displayed result.
According to a further prior art circuit of similar construction, as disclosed in U.S. Pat. No. 4,225,931, granted Sept. 30, 1980, the computer serves for calculating interpolation values for performing a further division within a signal period, i.e. for achieving smaller digital steps.
These prior art circuits can only be used in connection with the aforedescribed display device and equally are only provided for this purpose. It would not be possible to input the values which have been delivered by the computer directly to a servo mechanism of a processing machine or the like, since in such case the slightest false information, which results from the finite processing time of the counter, is not permissible. With the prior art circuit the display or indicator device is read by the human eye, which, if the signal scanning rate is of sufficient magnitude, cannot detect faulty displays at all because the display or indicator device always shows a stable value. However, there may in fact be present between two displayed values signal surges or jumps. These signal surges could render unstable the regulation and cause such regulation which is performed by a servomechanism to oscillate or hunt. If, with the prior art circuits, the counter has a certain counter state as a result of a preceding event and there now occurs a further event, then the counter needs a finite period of time for counting forwards or backwards to the new counter state which corresponds to this new event. This finite period of time may be in the order of 600 to 800 ns. During this time, the count or output state of the counter is unstable. With prior art circuits this does not constitute a disadvantage because the human eye averages the display shown at the display or indicator device. Since such possibility does not and must not exist if the computer output data is delivered to a servomechanism rather than to a display or indicator device, the prior art circuits cannot be used in this case.